We must resist this dark female lust, Betty, or it will swallow us up.
This quote, also in Act I, Scene three echoes Clive's earlier comment that he fears Africa "swallowing" him up. Clive connects the weakness of women with the perceived savageness of Africa. The notion that Betty's lust is "dark" implies an evil beyond the control of any woman. Clive insists that he and Betty must work together to resist the onslaught of evil forces. He does not even give Betty credit for her own choices, maintaining that Betty's identity is some combination of a female weakness and his ability to control that weakness.
Here, Clive also denies the truth of his family falling apart. By accepting Betty's infidelity as something that is beyond her control, he is able to deny that he has been betrayed. Clive attributes his family's indiscretions to larger forces in order to avoid the painful truth that he, as a father and husband, might have failed his family. He accepts no responsibility for Betty's wrongdoing. This quote is part of a larger lecture in which he hardly gives Betty an opportunity to speak. She only affirms his position that they have been victim to the inherent weaknesses of femininity.